The use of optical fiber for long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity continues to increase, there is a continuing need to utilize the bandwidth of existing fiber-optic cable more efficiently. An established method for increasing the carrying capacity of existing fiber cable is Wavelength Division Multiplexing (WDM). In this method, multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light and each light-wave-propagated information channel corresponds to light within a specific wavelength range or “band.”
In this specification, these individual information-carrying lights are referred to as either “signals” or “channels.” The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.”
An optical interferometer is a device that produces a periodic phase modulation of light within a range of wavelengths. When an optical interferometer is incorporated as a component within another optical apparatus, such as a wavelength division multiplexer or de-multiplexer, this periodic phase modulation may be utilized advantageously to produce periodic transmission, reflection, optical delay or polarization properties within optical signals or composite optical signals passed through the apparatus. However, the peak positions of the periodic optical properties associated with the interferometer must be precisely adjusted so as to align these peak positions to standard channel positions. This requires precise adjustment of the optical path length within the interferometer, often with a tolerance of just a few nanometers.
Various mechanical, thermo-optic, electro-optic or magneto-optic methods have been employed to provide adjustment or tuning capabilities to optical interferometers. Although these methods provide adequate tuning capabilities, they invariably add additional optical components, mechanical components (or moveable components) and/or electronic connections to the interferometer, thereby increasing the complexity and difficulty of fabricating and aligning the interferometer and potentially reducing the stability and optical throughput of the interferometer.
Accordingly, there remains a need for an improved interferometer that is easily adjustable. The improved interferometer should not incorporate unnecessary additional mechanical or electrical elements into the optical path within the interferometer and should not disturb or move any of the optical components disposed within or associated with this optical path. The present invention addresses such a need.